Annealing is a process to heat a metal structure and the like, hold it at a temperature and then cool the same to lower temperature. It is a heat treatment intended for improvement of plastic working properties, removal of residual internal stress, adjustment of crystal grain and the like. For annealing a metal structure formed of nickel (Ni), the structure is generally heated to around 700-900° C. and held for 1-2 hours, and then cooled down slowly. With this heat treatment, since nickel (Ni) is recrystallized, nanocrystal material of a crystal size of around 10 nm, or amorphous material of a further smaller crystal size, will have a larger crystal size of several μm-several ten μm. The relatively unordered initial structure is reconfigured by annealing, to arrive at a more stable state of equilibrium, and is essentially removed of all residual internal stress.
As an improved annealing method, Japanese Patent National Publication No. 2001-516812 discloses a method of heat treatment to obtain stable machinability under load. In this method, a wire coated with metal such as nickel (Ni) and cobalt (Co) using saccharine and 2-butyne-1,4-diol and the like is relatively mildly heat-treated to fabricate an elastic metal structure.
For a heat treatment temperature of this method, effective is a temperature 0-150° C. higher than the transformation temperature at which crystals of a metal material on the wire become larger. For example, for a wire coated with a metal material including nickel (Ni) and cobalt (Co), the effective temperature for heat treatment is 266-416° C. since the transformation temperature of the metal material is 266° C. (refer to FIG. 6). Therefore, in this method, the structure is heat treated at a temperature lower than the normal annealing temperature for nickel (Ni) of 700-900° C.
For a metal structure including nickel (Ni) and cobalt (Co), when annealing is applied at 330° C. for 10 minutes, nanocrystal material or amorphous material having an average grain diameter of 16 nm will have larger crystals to become crystal material with an average grain diameter of 78 nm. Consequently, due to annealing, yield strength, elastic modulus and temperature stability of the metal structure are modified.
As the semiconductor technology advances, importance of a contact probe, which is necessary to test a circuit formed on a semiconductor substrate or the like, increases. Since the contact probe is pressed against a circuit of a semiconductor substrate or the like for usage, it has a spring capability to increase the reliability of connection with the circuit as well as not to damage the circuit. Therefore, the contact probe is required to have the property of being unlikely to become brittle and damaged while having high hardness. Further, since the contact probe is repeatedly used, it is required to have a property to return to its original shape when load is removed after the test. Thus, it is required for the contact probe that the amount remaining as deformation after load is removed (hereinafter referred to as “creep amount”) is small, and that spring load which occurs when the spring achieves a predetermined stroke does not change with loading time. Therefore, it is necessary to use a metal structure having superior creep resistance. More specifically, it is required to maintain creep resistance even when a semiconductor substrate to be tested is at a temperature of around 50-125° C. or higher.